1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device constituted of liquid crystal mixed with a polymer, and a projector using it, and also relates to a method for manufacturing the LCD device.
2. Related Background Art
In recent years, the liquid crystal display device has been employed in various apparatuses, as represented by a direct vision display of a diagonal length of 10 inch or more. Further, according to the popularization of computers, the liquid crystal projector of high resolution and brightness, which projects enlarged images to a screen from a liquid crystal panel combined with an optical system, has begun entering into a market as a practical product.
The liquid crystal materials used in such a liquid crystal panel are of TN (twisted nematic) type, and give rapid response and high contrast compared with those of STN (supertwisted nematic) type mainly used before, to display images of higher quality.
A display device using a TN liquid crystal, however, has a limited brightness because of considerable light loss due to the polarizers, which defect is conspicuous in a projector type liquid crystal display device which requires a high brightness.
To solve this problem, there has been proposed a liquid crystal display device using a "polymer network liquid crystal (PNLC)" in which a TN liquid crystal material has been distributed in a polymer network. In this device, when an electric voltage is applied, the TN crystal comes to have a refractive index approximately the same as that of polymer network, and light passes through with a high transmittance, and when the electric voltage is not applied, TN crystal takes a random alignment to have a different refractive index from that of polymer network, and thus incident light is scattered, to display black. The display device using a "polymer network liquid crystal" does not use polarizers, and thus is a display of higher light efficiency, that is, a brighter display, than the TN liquid crystal display device.
The term "polymer network liquid crystal" used in this specification may be also applied to liquid crystals having similar structural features, such as a polymer dispersed liquid crystal (PDLC) and liquid crystals of spongy structure.
To further enhance the high light efficiency of "polymer network liquid crystal", it is effective to use the active matrix substrate of reflection type. This is because the reflection type substrate can place active elements beneath the reflective electrode which can also serve as a shield against light, and thus permit the aperture to open nearly to its full range. Therefore, even if the size of pixel is reduced, it has a potential to maintain the same light exploitation efficiency, in contrast with the transmission type panel.
When a reflection type "polymer network liquid crystal" or a "polymer/liquid crystal compound membrane" or a "liquid crystal dispersed in polymer" is manufactured, active elements are embedded under the reflective electrode of the reflective active matrix substrate, and polymer and liquid crystal materials are injected and enclosed into the space between the substrate and transparent electrode, followed by exposure to ultra-violet (UV) light to produce a liquid crystal panel.
The above manufacture of a "polymer dispersed liquid crystal" is described in Japanese Patent Application Laid-Open No. 5-61016. According to the disclosure, an acrylate composition which can polymerize under UV rays (Dalocure 1116 provided by Merck is used as a photo-polymerization initiator) and a liquid crystal composition (E 8 provided by BDH) are blended to evenness, and the mixture is injected into a glass cell having ITO electrodes and exposed to UV rays (1 mW, 500 seconds) to produce a polymer dispersed liquid crystal (PDLC). By varying the content of the liquid crystal composition in PNLC between 65 to 75 weight% with respect to the total weight of polymer matrix and the liquid crystal composition, samples of various particle size were prepared.
However, it has been found that the liquid crystal panel produced by conventional UV exposure is often afflicted with uneven brightness. Particularly, when the whole surface of liquid crystal area is irradiated with parallel UV light, reduction of reflected light intensity, or light transmittance is observed at the center and its periphery of liquid crystal panel, in a concentric pattern. With a three-plate liquid crystal panel for color display, there occurs a concentric unevenness in hue. This is accounted for by uneven polymerization of the polymer or uneven size of liquid crystal particles, which may be caused by stresses generated during polymerization reaction. As another cause, one can think the influence of the seal during polymerization reaction, which may differ according to the distance from the seal.
When polymerizing monomer substance in a closed, plane region, uniform initiation of polymerization hardly occurs even though the energy required for polymerization initiation is supplied uniformly all over the area, and uncontrollable regional irregularity will develop, at least microscopically, due to the molecular level fluctuation of polymerizable materials.
In addition, as with the production of liquid crystal display panels, when the liquid crystal/prepolymer mixture is injected into a cell constituted of different materials such as an Si substrate on which elements are formed, an opposite glass substrate and a sealant, many factors including process factors disturb the uniformity and affect the progression of polymerization and it is very difficult to form a uniform network structure in the area uniformly and with good reproducibility.
Further, the conventional liquid crystal panel was found to have a problem in stability. Namely, it becomes often unstable because of the presence of a liquid crystal phase not reacted during UV exposure.